Okay, let's break down the VFD feeder calculation based on the information you've provided and the relevant sections of the NEC. You're on the right track in your thinking.

Understanding the Problem You have a VFD with a known input VA rating. The specific pump connected to the VFD is unknown. You're calculating the feeder load using NEC 220.40.

Key NEC Sections and How They Apply 220.40 (General)

This section directs you to Article 430 (Motors, Motor Circuits, and Controllers) for motor loads.

220.50 (Appliances and Electronic Equipment)

This section says to apply a load factor if available or use 100% of the rated load for appliances, unless otherwise permitted. This doesn't override the specific rules for motors and VFDs found in Article 430.



Article 430 (Motors, Motor Circuits, and Controllers)

This is the governing article for your situation.

430.24 (Several Motors or Motor(s) and Other Load(s))

This deals with situations where you have multiple motors or a motor and other non-motor loads on the same feeder. It's not your primary focus here because you're calculating the feeder for one VFD/pump.

430.25 (Motor-Driven Continuous Duty Loads)

This is for when you have motor-driven equipment that runs continuously.

430.26 (Motors on General-Purpose Branch Circuits)

This covers motor applications when connected to a branch circuit supplying other equipment. This is for smaller motor loads.

430.122 (Conductor Rating and Size - Adjustable-Speed Drive Systems)

This is your primary focus. It explicitly addresses adjustable-speed drive systems (VFDs). It requires that feeder conductors supplying the entire adjustable-speed drive system be sized no less than 125% of the input current rating marked on the drive.

Why 430.122 is Key (and Why You Don't Need the Pump Information)The critical point here is that 430.122 focuses on the input to the VFD. The code recognizes that the output of the VFD (to the motor) is variable, but the input is what dictates the maximum demand the VFD will place on the feeder. Essentially, the VFD is treated as a piece of electrical equipment with a certain input demand.Therefore, you're correct:

You don't need to know the pump's horsepower or full-load amps (FLA) to size the feeder. The VFD's input VA (or the resulting calculated input current) is the only information you require, given that the pump is as-yet undefined.

Your feeder conductor ampacity must be at least 125% of the VFD's input current.

Calculation Steps1.

Determine VFD Input Current

Divide the VFD's input VA by the input voltage to get the input current (I = VA / V). Be sure to use the correct input voltage for your system. If it's a single-phase VFD connected to a 240V system, use 240V.2.

Apply the 125% Factor

Multiply the VFD input current by 1.25. This is the minimum ampacity your feeder conductors must have.
3.

Select Conductor Size

Choose conductors from NEC Table 310.16 (or appropriate table) that have an ampacity equal to or greater than the value calculated in step 2, taking into account conductor insulation type and termination temperature ratings.
4.

Size Overcurrent Protection

Size the overcurrent protective device (circuit breaker or fuses) according to NEC Article 430 Part III (Overload Protection) and Part IV (Short-Circuit and Ground-Fault Protection). The overcurrent protection should not be less than 125% of the VFD input current and should be able to handle the VFD inrush current.

ExampleLet's say your VFD has a single-phase input rating of 24 kVA and is connected to a 240V system.1.

Input Current

24,000 VA / 240 V = 100 amps2.

125% Factor

100 amps 1.25 = 125 amps3.

Conductor Size

You'd need to select conductors with an ampacity of at least 125 amps. For example, using THHN copper conductors with 75°C terminations, you would need at least a #1 AWG conductor (rated for 130 amps).4.

Overcurrent Protection

You'd need an overcurrent device rated for at least 125 amps. The appropriate size will be determined by the VFD manufacturer's recommendation.

Important Considerations

VFD Manufacturer's Instructions

Always follow the VFD manufacturer's installation instructions. They may have specific requirements or recommendations for conductor sizing and overcurrent protection.

Voltage Drop

Calculate voltage drop for the feeder to ensure it's within acceptable limits, especially for long runs. Oversizing the conductors may be necessary to reduce voltage drop.

Grounding

Ensure proper grounding of the VFD and the motor it controls.

Harmonics

Be aware that VFDs can generate harmonics, which can affect power quality. Consider using harmonic filters if necessary.

Future Motor

While you don't need the pump's information now, consider the potential future needs. If you anticipate upgrading to a larger motor/pump in the future, it might be wise to size the feeder accordingly now to avoid having to replace it later. You can always size up, but never down.

Code Editions

Always refer to the most recent edition of the NEC adopted in your jurisdiction.

In summary: You are correct in focusing on 430.122 and using the VFD input VA to determine your feeder conductor size. The pump information is not* required at this stage. Just make sure you adhere to all applicable NEC requirements and the VFD manufacturer's instructions.Good luck with your calculation! Flag for review